1. Field of the Invention
This invention relates to a radiation image storage panel using a stimulable phosphor, and more particularly to a radiation image storage panel for recording and reproducing a radiation image using a stimulable phosphor which stores radiation energy and emits light upon stimulation thereof.
2. Description of the Prior Art
As is well known in the art, a photographic method using a silver salt such as radiography in which an X-ray film having an emulsion layer comprising a silver salt is used in combination with an intensifying screen has generally been employed to obtain a radiation image. Recently, because of problems such as the shortage of silver resources, a method of obtaining a radiation image without using a silver salt has been desired.
An example of such a method is disclosed in U.S. Pat. No. 3,859,527. In the method of the patent, there is used a radiation image storage panel comprising a stimulable phosphor which emits light when stimulated by an electromagnetic wave selected from among visible light and infrared rays after exposure to a radiation (The term "radiation" as used herein means electromagnetic wave or corpuscular radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, high-energy neutron rays, cathode rays, vacuum ultraviolet rays, ultraviolet rays, or the like.). The method comprises the steps of (i) causing the stimulable phosphor of the panel to absorb a radiation passing through an object, (ii) scanning the panel with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the panel as light emission, and (iii) electrically converting the emitted light into an image.
In this connection, it is well known in the art that in conventional radiography in which an intensifying screen is used in combination with an X-ray film, the sharpness of the image obtained depends upon the degree of spread of the spontaneous light emitted by the phosphor in the intensifying screen. In contrast to this, in the above-mentioned method for recording and reproducing a radiation image utilizing the stimulability of a stimulable phosphor, the sharpness of the image obtained does not depend upon the degree of spread of the light emitted by the stimulable phosphor in the panel, but depends upon the degree of spread of the stimulating rays in the panel. The reason for this can be explained as follows. In the above-mentioned method for recording and reproducing a radiation image, the radiation image stored in the panel is taken out of the panel sequentially as mentioned above. Therefore, all of the light emission caused by the stimulating rays for a certain period (ti) is desirably detected as the output of a certain picture element (xi, yi) on the panel which is exposed to the stimulating rays during the period (ti). Where the stimulating rays spread in the panel due to scattering or the like and stimulate the phosphor surrounding the picture element (xi, yi) in addition to the picture element (xi, yi), the output for an area broader than the picture element (xi, yi) is detected as the output of the picture element (xi, yi). Accordingly, if the light emission caused by the stimulating rays during the period (ti) is only that emitted by the picture element (xi, yi) which has been exactly exposed to the stimulating rays during the period (ti), the emitted light does not affect the sharpness of the image obtained no matter how the emitted light spreads in the panel.
The radiation image storage panel employed in the above-mentioned method for recording and reproducing a radiation image has at least a fluorescent layer comprising a proper binder and a stimulable phosphor dispersed therein. Although the fluorescent layer itself can be a radiation image storage panel when the fluorescent layer is self-supporting, the fluorescent layer is generally provided on a proper substrate to form a radiation image storage panel. Further, a protective layer for physically and chemically protecting the fluorescent layer is usually provided on the surface of the fluorescent layer to be subject to exposure. Furthermore, a primer layer is sometimes provided between the fluorescent layer and the substrate to closely bond the fluorescent layer to the substrate. In the conventional radiation image storage panel having such a structure, the stimulating rays broadly spread in the panel due to irradiation in the fluorescent layer, halation in the protective layer, the primer layer or the substrate, or the like. Therefore, an image of high sharpness cannot be obtained by the conventional radiation image storage panel.
As a radiation image storage panel in which the above-mentioned defect in the conventional radiation image storage panel is reduced, a radiation image storage panel colored with a colorant was invented and an application for patent on the panel was filed (see U.S. Pat. No. 4,394,581). The radiation image storage panel disclosed in the application has a fluorescent layer comprising a binder and a stimulable phosphor dispersed therein and is characterized by being colored with a colorant so that the mean reflectance of the panel in the wavelength region of the stimulating rays for the stimulable phosphor is lower than the mean reflectance of the panel in the wavelength region of the light emitted by the stimulable phosphor upon stimulation thereof. In the radiation image storage panel, the spread of the stimulating rays in the panel can be controlled by the absorption of the stimulating rays by the colorant and as a result, an image having markedly improved sharpness can be obtained.
It is described in the above-mentioned U.S. Pat. No. 4,394,581 that either an organic colorant or an inorganic colorant can be employed as the colorant. Actually, in the case wherein either an organic colorant or an inorganic colorant is employed, a radiation image storage panel which provides an image of high sharpness can be obtained if the colorant employed absorbs the stimulating rays. However, as a result of subsequent studies regarding the radiation image storage panel colored with a colorant, it was found that other image characteristics of the panel such as granularity and contrast varied considerably depending upon the colorant employed in the panel.
In the colored radiation image storage panel disclosed in the above-mentioned U.S. Pat. No. 4,394,581 the panel colored with an organic colorant generally provides an image of better granularity than the panel colored with an inorganic colorant. This is because the particle size of an organic colorant is generally very small in comparison with the particle size of an inorganic colorant. Therefore, from the viewpoint of the granularity of the radiation image storage panel, it is preferable to employ an organic colorant as the colorant of the panel. However, when the organic colorants described in the above-mentioned U.S. Pat. No. 4,394,581 are employed as the colorant, only a radiation image storage panel which provides an image of low contrast can be obtained. This is because all of the organic colorants described in the above-mentioned U.S. Pat. No. 4,394,581 exhibit light emission of longer wavelength (generally in the red to infrared region) than that of the stimulating rays (generally in the wavelength region of 500 to 800 nm depending upon the stimulable phosphor employed) when exposed to the stimulating rays, and at least a part of the light emission is reproduced as noise observed throughout the radiation image storage panel. In Example 1 of the above-mentioned U.S. Pat. No. 4,394,581, is described a method for recording and reproducing a radiation image comprising the steps of (i) exposing a radiation image storage panel having a fluorescent layer composed of BaFBr:Eu.sup.2+ phosphor (stimulable phosphor) and colored with Zapon Fast Blue 3G (organic blue colorant, manufactured by Hoechst AG.) to X-rays, (ii) scanning the panel with a He-Ne laser beam (633 nm), (iii) detecting the light emitted by the fluorescent layer of the panel by means of a photosensor to convert the light to an electrical signal, and (iv) converting the electrical signal to an image signal by means of a reproduction device to obtain an image on a display device. With particular reference to the above-mentioned method for recording and reproducing a radiation image, the lowering of the contrast of the reproduced image due to the light emission of the organic colorants caused by the stimulating rays will be described in detail hereinbelow.
As shown in FIG. 1, a radiation image storage panel 13 having a fluorescent layer composed of BaFBr:Eu.sup.2+ phosphor and colored with Zapon Fast Blue 3G is exposed to X-rays emitted by an X-ray source 11 passing through an object 12, and thereafter, the panel 13 is scanned with a He-Ne laser beam emitted by a He-Ne laser source 14. By the stimulation with the He-Ne laser beam, the fluorescent layer of the panel 13 emits light. The light emitted by the fluorescent layer of the panel 13 is detected and converted to an electrical signal by a photosensor 15. Then, the electrical signal obtained is converted to an image signal by a reproduction device 16, and a visible image is displayed by a display device 17. In such a method for recording and reproducing a radiation image, a filter 18 is placed between the panel 13 and the photosensor 15. The filter 18 is used for cutting the noise constituted by the He-Ne laser beam reflected by the panel 13, thereby only transmitting the light emitted by the fluorescent layer of the panel 13. As shown in FIG. 2, the BaFBr:Eu.sup.2+ phosphor constituting the fluorescent layer of the panel 13 has an emission spectrum in the near ultraviolet to green region (the emission spectrum has a peak in the neighborhood of 400 nm), and the wavelength of the He-Ne laser beam is 633 nm. Therefore, an ultraviolet to short wavelength visible light-transmitting filter which selectively transmits light of shorter wavelength than green light is employed as the filter 18. The ultraviolet to short wavelength visible light-transmitting filter transmits almost no long wavelength visible light, and accordingly, the He-Ne laser beam having a wavelength of 633 nm reflected by the panel 13 is completely cut by the filter. However, as illustrated by curve-a of FIG. 3 representing the spectral transmittance curve, all ultraviolet to short wavelength visible light-transmitting filters have high transmittance for infrared rays. On the other hand, as shown by curve-b of FIG. 3 representing the emission spectrum, Zapon Fast Blue 3G color the fluorescent layer of the panel 13 exhibits light emission in the red to infrared region when exposed to the He-Ne laser beam. Accordingly, as is clear from FIG. 3, when the panel 13 is exposed to the He-Ne laser beam, a part of the light emission (light emission in the infrared region) of Zapon Fast Blue 3G passes through the filter together with the light emission of the BaFBr:Eu.sup.2+ phosphor. The light emission of the colorant passing through the filter is reproduced as a noise observed throughout the panel and as a result, the contrast of the image obtained is lowered.